The present invention relates to disc or butterfly type valve in which a disc is pivotally disposed within a generally cylindrical housing for pivotal movements of the disc between an open position, in which the disc is generally parallel to the axis through the housing, and a closed position in which the disc is transverse to the axis of the housing.
A large variety of disc valves are presently in use. One particularly advantageous construction is disclosed and claimed in U.S. Pat. No. 3,442,488, the disclosure of which is incorporated herein by reference. To briefly summarize the operation of the valve disclosed in that U.S. Patent, the disc is rotatable about an eccentric pivot axis to move it into and away from sealing engagement with the internal or sealing surface of an inclined annular valve seat. The sealing surface of the valve seat defines an conical surface whose axis forms an acute angle with the axis of the valve housing. The disc pivot axis is perpendicular to and offset from the housing axis to define a larger and a smaller disc and sealing surface segment. The configuration of the sealing surface and of the disc are such that when the disc is tilted into its closed position its sealing element or seal ring is wedged into sealing engagement with the valve seat. Wear of the sealing surfaces or the seal ring from predominantly sliding engagement is thereby reduced or eliminated. Further details of the construction of such a valve are set forth in the referenced patent.
When the seal ring of the disc is made from an inelastic material, such as metal, it has proven difficult to assure a tight fit between the seal ring and the valve seat over the full circumference of the former. Failure to assure such a tight seat results in valve leakage when the disc is closed. In some applications, particularly when the valves are used in the chemical and petrochemical processing industries an absolute seal is mandatory. Even the slightest leakage across the closed disc cannot be tolerated since the fluid media on the upstream and on the downstream sides of the disc must, in many such applications, be maintained fully separated to prevent explosions.
To assure a complete seal it was heretofore necessary to machine the sealing element, assemble it with the disc and install it in the valve housing with the utmost care. This has proved particularly burdensome and difficult with valves of large diameters, some of them reaching diameters up to three meters. This in turn resulted in substantial cost increases and made such valves relatively expensive to produce.
Furthermore, particularly when the seal ring of the disc is constructed of metal the contact between it and the valve seat could lead to surface damage when foreign objects or particles are present on the seat. Such surface damage, normally scratches or indentations in the valve seat surface, could lead to leakage when the disc is closed.
Once leakage due to valve seat surface damage has occurred it can only be rectified by repairing the valve. Normally this involves the removal of the valve from the line, its transportation to the plant and expensive repair work. While the valve is removed from the pipeline the latter is inoperative unless a replacement valve is available. Thus, valve repair can result in down-times of expensive processing plants and cause heavy losses.